Summary:
EmrB is a multidrug efflux protein. Expression of emrB has been shown to confer resistance to hydrophobic compounds such as carbonyl cyanide m-chlorophenylhydrazone, tetrachlorosalicylanilide, organomercurials and nalidixic acid [Lomovskaya92]. Disruption of emrB results in increased sensitivity to these compounds [Lomovskaya92]. EmrB has also been implicated in conferring resistance to thiolactomycin [Furukawa93]. EmrB is a member of the major facilitator superfamily, suggesting that it functions as a multidrug/proton antiporter.

emrB is co-transcribed with emrA, which encodes a member of the membrane fusion protein family [Lomovskaya92, Dinh94]. Genetic evidence has suggested that emrA and emrB may function together in conferring resistance. This suggests that EmrA and EmrB may form part of a complex mediating multidrug efflux. Transcriptional fusions have suggested that expression of ermA and emrB is negatively regulated by EmrR, whose gene is cotranscribed with emrA and emrB [Lomovskaya95]. CCCP, nalidixic acid and other substrates of EmrB have been shown to induce expression of emrB.

Summary:
The EmrAB-TolC Drug Efflux System consists of the EmrA membrane fusion protein, the EmrB drug efflux pump (a member of the major facilitator superfamily), and TolC, the outer membrane protein that extrudes the drugs to the extracellular environment. One possible model of transport might be one in which EmrA and EmrB form a stable complex, possibly via their membrane-spanning leucine zipper motifs. The β-sheet domain of EmrA may be positioned above EmrB at the membrane surface with the α-helices of EmrA radiating across the periplasm to interact with TolC when triggered by the binding of drugs to the β-sheet domain of EmrA [BorgesWalmsley03]. Electron microscopy of reconstituted EmrAB complex suggests that the physiological form of EmrAB is a dimer [Tanabe09].